Early Earth zircons formed in residual granitic melts produced by tonalite differentiation

The oldest geological materials on Earth are Hadean (>4 Ga) detrital zircon grains. Their chemistry and apparently low Ti-in-zircon temperatures (≤700 °C) are considered to be inconsistent with crystallization in a magma of the tonalite-trondhjemite-granodiorite (TTG) suite, although these are the dominant Archean (4.0–2.5 Ga) silicic rocks. Using a new data set of trace element contents in zircons from Paleoarchean Barberton TTGs (South Africa) and thermodynamic modeling, we show that these zircons have crystallized at near-solidus conditions from a compositionally uniform granitic melt. This melt is residual from the crystallization of a less evolved (tonalitic) parent and thereby shows major and trace element compositions different from bulk TTG rocks. A global compilation reveals that most Hadean detrital and Archean TTG-hosted grains share a peculiar zircon trace element signature that is distinct from the chemical trends defined by Phanerozoic zircons. Our model shows that the low Ti contents of early Earth zircons reflect crystallization at higher temperatures (720–800 °C) than initially inferred due to lower modeled TiO2 activity in the melt relative to previous estimates. We therefore propose that near-solidus zircon crystallization from a chemically evolved melt in a TTG-like magmatic environment was the dominant zircon-forming process on the early Earth.

Hadean zircon formed due to hydrated ultramafic protocrust melting

Hadean zircons, from the Jack Hills (Western Australia) and other localities, are currently the only window into the earliest terrestrial felsic crust, the formation of which remains enigmatic. Based upon new experimental results, generation of such early crust has been hypothesized to involve the partial melting of hydrated peridotite interacting with basaltic melt at low pressure (<10 km), but it has yet to be demonstrated that such liquids can indeed crystallize zircons comparable to Jack Hills zircon. We used thermodynamic and geochemical modeling to test this hypothesis. The predicted zircon saturation temperatures of <750 °C, together with the model zircon Th, U, Nb, Hf, Y, and rare earth element (REE) contents at 700 °C, δ18OVSMOW (Vienna standard mean ocean water) signatures, and co-crystallizing mineral assemblage were compared to those of the Jack Hills zircon. This comparison was favorable with respect to crystallization temperature, most trace-element contents, and mineral inclusions in zircon. The discrepancy in δ18OVSMOW signatures may be explained by hotter conditions of Hadean protocrust hydration. Our work supports the idea that felsic magma generation at shallow depths involving a primordial weathered ultramafic protocrust and local basaltic intrusions is indeed a viable mechanism for the formation of felsic crust on early Earth.

Sedimentary Environmental Evolution of the Western Taiwan Shoal Area since the Late Pleistocene

A new pollen analysis and major and trace element contents were conducted on a 40 m long gravity core recovered from the Taiwan Shoal (sand ridges), south of the Taiwan Strait, beginning in the Late Pleistocene. The changes in the pollen assemblage and concentration represent the climate change around the Taiwan Shoal and the strength of the Zhe-Min Coastal Current, whereas variations in major and trace element contents can imply the source of the sediments in the Taiwan Shoal, which are correlated with the rise or fall of the sea level with increased marine dinoflagellate cysts. The interval of 40–30 m was characterized by high pollen and spore concentrations, and evergreen Quercus was dominant taxon, which indicates a warm sedimentary environment, and the surrounding area of the Taiwan Shoal were covered by a tropical and subtropical broad-leaved forest. There were no pollen and spores from 30–24 m, which indicates a strong hydrodynamic sedimentary environment, and most of the Taiwan Shoal might have been experience subaerial exposure. The interval of 24–17 m was characterized by the reappearance of pollen and spores, as well as marine dinoflagellate cysts and foraminifera, suggesting the climate was warm and wet in the study area and an apparent marine sedimentary environment with relatively high sea level. Deciduous Quercus dominated the interval of 17–12 m, which indicated that the climate was relatively cool, corresponding to the end of Marine isotope stages3 (MIS3) to the Last Glacial Maximum accompanied by weathering and denudation. Above 12 m, the low pollen concentration with increased marine dinoflagellate cysts and foraminifera abundance suggested a marine sedimentary environment in the Taiwan Shoal. The high concentrations in Pinus corresponds to Holocene high sea level.

Evolution of a hydrothermal ore-forming system recorded by sulfide mineral chemistry: a case study from the Plaka Pb–Zn–Ag Deposit, Lavrion, Greece

Laser ablation-inductively coupled plasma-mass spectrometry and electron-probe microanalysis were used to investigate the trace-element contents of sphalerite, chalcopyrite and pyrite from the Plaka Pb–Zn–Ag deposit. Using petrographic observations, the analytical results could be linked to the temporal evolution of the Plaka ore-forming system. Sphalerite chemistry reliably records the temperature and fS2 evolution of the system, with estimated formation temperatures reproducing the microthermometric results from previous fluid-inclusion studies. Chalcopyrite chemistry also shows systematic variations over time, particularly for Cd, Co, Ge, In, Sn and Zn concentrations. Measurable pyrite was only found in association with early high-temperature mineralisation, and no clear trends could therefore be identified. We note, however, that As and Se contents in pyrite are consistent with formation temperatures estimated from co-existing sphalerite. Statistical analysis of the sphalerite data allowed us to identify the dominant geological controls on its trace-element content. The three investigated factors temperature, fS2, and sample location account for > 80% of the observed variance in Mn, Fe, Co, Ga, Ge, In, Sb and Hg concentrations, and > 60% of the observed variance in Cd and Sn concentrations. Only for Cu and Ag concentrations is the explained variance < 50%. A similarly detailed analysis was not possible for chalcopyrite and pyrite. Nevertheless, comparison of the results for all three investigated minerals indicates that there are some systematic variations across the deposit which may be explained by local differences in fluid composition.

Chemical Characteristics of Ore-Bearing Intrusions and the Origin of PGE–Cu–Ni Mineralization in the Norilsk Area

The composition of the parental magmas of Cu–Ni deposits is crucial for the elucidation of their genesis. In order to estimate the role of magma in ore formation, it is necessary to compare the compositions of silicate rock intrusions with different mineralization patterns, as observed in the Norilsk region. The rock geochemistry of two massifs located in the same Devonian carbonate rocks—the Kharaelakh intrusion, with its world-class platinum-group element (PGE)–Cu–Ni deposit, and the Pyasinsky–Vologochansky intrusion, with its large deposit—was studied. Along with these massifs, the Norilsk 2 massif with noneconomic mineralization intruded in the Ivakinskaya–Nadezhdinskaya basalts was studied as well. Their settings allow the estimation of the parental magma composition, taking into account the possible assimilation of host rocks. Analyses of 39 elements in 97 samples demonstrated the similarity of the intrusions in terms of their major components. The Pyasinsky–Vologochansky intrusion contains the highest trace element contents compared with the Kharaelakh and Norilsk 2 massifs, evidencing its crystallization from evolved parental magma. No influence of host rocks on the silicate rock compositions was found, except for narrow (1–2 m) endo-contact zones. There is no correlation between the mineralization volume and the rock compositions of the studied intrusions. It is assumed that the intrusions were formed from one magma crustal source irregularly rich in sulfur (S). This source inhomogeneity in terms of the sulfur distribution resulted in deposits of varying sizes. The magmas served as a transporting agent for sulfides from deep zones to the surface.

U–Pb and Hf isotope study of detrital zircon and Cr-spinel in the Banavara quartzite and implications for the evolution of the Dharwar Craton, south India

The Sargur Group has been considered to be the oldest group (>3.0 Ga) in the Archaean sequence of the Dharwar Craton in south India, whereas the rocks of the Dharwar Supergroup are younger (between 3.0 and 2.55 Ga). The supracrustal units of the Sargur Group were deposited during the Archaean period. The Banavara quartzite forms part of the supracrustal Sargur Group and contains significant amounts of chromian spinel (Cr-spinel). Here, U–Pb and Hf isotopes of detrital zircons are integrated with compositional data and X-ray refinement parameters for Cr-spinels to decipher the provenance of the metasediments. Zircons show an age spectrum from 3.15 to 2.50 Ga, and juvenile Hf isotopic compositions (ϵHf = +0.8 to +6.4) with model ages between 3.3 and 3.0 Ga. Major- and trace-element contents of the Cr-spinels do not resemble those in the Sargur ultramafic rocks, but resemble well-characterized Archaean anorthosite-hosted chromites. Cr-spinel trace-element signatures indicate that they have undergone secondary alteration or metamorphism. X-ray refinement parameters for the Cr-spinels also resemble the anorthosite-hosted chromites. We conclude that the detrital minerals were probably derived from gneissic and anorthositic rocks of the Western Dharwar Craton, and that the Sargur Group sequences have experienced a younger (2.5 Ga) metamorphic overprint.

On the article “Weathered potassic volcanic rocks as protoliths of the hematitic phyllites of the southern Serra do Espinhaço (MinasGerais)”: a discussion

Chaves and Knauer (2020) have presented three new whole-rock chemical analyses of phyllitic hematite, a unique metamorphic rock of the southern Serra do Espinhaço. Based on their three samples and a selection of other three samples from the literature, Chaves and Knauer have proposed that the geochemical uniqueness of the rock – i.e., high contents of K2O, Al2O3 and Fe2O3, and depletion in SiO2 – would represent a weathered, feldspathoid-rich alkaline basalt. This contribution is a discussion of their new data, the trace-element contents of which are at odds with those of a potassic, mantle-derived volcanic protolith for the hematitic phyllite. Its Nb/Th ratios of ~3 and chondrite-normalised La/Yb ratios of ~9–17, for instance, are typical of the continental crust. We also point out aspects that escaped the attention of Chaves and Knauer (2020), one of which is the ubiquitous occurrence of tourmaline in the hematitic phyllite.